Climate Dynamics (PCC 587): Hydrologic Cycle and Global Warming D A R G A N M. W. F R I E R S O N U N I V E R S I T Y O F W A S H I N G T O N, D E P A R T M E N T O F A T M O S P H E R I C S C I E N C E S 1 2-1 - 0 9 Recent Global Temperatures Source: GISS 1
Different Latitude Bands Tropics vs NH/SH extratropics: When temperatures go up in the tropics, global water vapor content goes up a lot Tropical Water Vapor Content Models can simulate changes in water vapor content well, if they re given the SSTs Model (solid) vs obs (dashed): Almost all plots today (including this) are from Held and Soden, J. Climate, 2006 2
Changes in Water Vapor Content Changes in water vapor content go essentially as fixed relative humidity Warmer oceans (e.g., due to El Nino) more moisture Global warming simulations say the same thing Moisture content goes up similar to fixed relative humidity 7% per degree warming There are important deviations from this locally, but it doesn t change the global mean picture Changes in Water Vapor in AR4 Simulations Changes in water vapor content (% increase) versus temperature change (K) for change over 21 st century (A1B scenario): Water vapor increases by 10-25% with warming. Spread among models is mostly due to spread in amount of warming. 3
How about precipitation? Precipitation doesn t necessarily increase when water vapor increases Can consider this from two perspectives: what determines evap or what determines precip Surface budget perspective: evaporation = solar net surface LW sensible heat flux Incoming solar radiation limits evaporation to some extent Atmospheric energy budget perspective: precipitation = net radiative cooling of atmosphere sensible heat flux How easily the atmosphere can radiate away heat limits precipitation Precipitation Changes with Warming Models show approximately 2% increase in global precipitation per degree warming Significantly less than water vapor content increase 4
So far Global moisture content increases quickly with warming Up to 25% by 2100 One of the largest responses with warming Global precipitation increases much less quickly Only up to around 5% by 2100 So the Earth will get somewhat wetter but what about the distribution of the precipitation changes? Next: why wet gets wetter and dry gets drier We ll start zonally averaged, and move on to more local changes Moisture Transports Moisture content increases rapidly. How about moisture transports? Northward moisture flux in current climate: Equatorward moisture flux by Hadley cell, poleward flux by midlatitude eddies 5
Moisture Flux Changes Moisture content increases faster than most circulation changes Change in moisture flux (solid line) looks like mean moisture flux Rough guess of moisture flux change (dashed line): Same circulation, increased moisture content Moisture Flux Changes Can see evidence for changes in circulations from difference from simple prediction Poleward shift of baroclinic eddies Weakening of NH midlatitude circulation 6
Implications for Precipitation More moisture fluxed into deep tropics and high latitudes Precipitation will strongly increase in those places Evaporation and moisture flux increase there More moisture fluxed out of subtropics This reduces the precipitation Evaporation changes offset somewhat Prediction of P-E The moisture flux argument most directly relates to P-E (this is equal to moisture transport) Clear increase in deep tropics & High latitudes Clear decrease in subtropics Also see clear poleward shifts 7
Change in Precipitation Change in precipitation in global warming simulations Wet gets much wetter, dry gets a bit drier (although this can be a large percentage change relative to the current value in arid regions) Note also poleward shifts relative to simple prediction Predictions of P-E Actual P-E change (top) vs simple prediction as proportional to P-E (bottom) (Here red = moistening, blue = drying: future plots will be opposite) Can see more drying over many continents (SW US/Mexico, Southern Africa, Northern Europe) Some of this is clearly associated with poleward shifts of circulations But there are kind of fundamental problems over land what is up? 8
Complications of Land Surfaces Over a whole continent, precipitation minus evaporation must be greater than zero This is equal to the runoff (and there can t be run-on into a given water basin) Simple argument about moisture fluxes doesn t necessarily respect this Arid land regions have P ~ E Everything that precipitates later evaporates Near surface humidity can be small and is very variable One wouldn t expect moisture content over arid regions to increase with warming Complications of Simple Argument: Arid Land Simple argument thus predicts an increase in moisture flux over even the driest land surface Can see this in previous predictions Arid land: what happens? In reality, P-E would remain close to zero over arid land How P changes over these locations is then determined by changes in moisture fluxes (which may increase, but won t necessarily ) Let s next examine changes in modeled precip 9
Annual Mean Precipitation For reference, the precip climatology from TRMM (1998-2008) Modeled Precipitation Changes Seasonal percentage change in precip from IPCC AR4 10
Modeled Precipitation Changes Alternative: # of models predicting drying vs wetting Modeled Precipitation Changes Alternative: # of models predicting drying vs wetting 11
Changes in Land Precipitation Note that many arid continental regions show decreases in precipitation Even though increased moisture flux argument doesn t constrain those locations Decrease in soil moisture important? Let s take a closer look at land precipitation changes all around the globe North America Predicted N. Amer. rainfall change (annual & seasonal) 12
SW North American Drying Seager et al 2007: Model Projections of an Imminent Transition to a More Arid Climate in SW North America Poleward shift of eddies is key to their explanation of this decrease Europe Modeled European precip changes 13
Africa Modeled African precip changes Asia Modeled Asian precipitation changes 14
South America Modeled South American precip changes Australia Modeled Australian precipitation changes 15